1. Field of the Invention
The present invention relates to a wireless receiving device, and more particularly, to a wireless receiving device used in a mobile communications system, for example, at its base station.
2. Description of the Background Art
In wireless receiving devices, Automatic Gain Control (AGC) has been widely employed for converging the level of a received signal having significantly been attenuated during propagation, to a desired level (for example, see Japanese Patent Laying-Open No. 2003-158557).
FIG. 3 is a functional block diagram showing a configuration of a wireless receiving device used in a mobile communications system (at its base station or mobile station) such as PHS (Personal Handyphone System), as an example of a conventional wireless receiving device employing AGC.
Referring to FIG. 3, the wireless receiving device includes an antenna 1, a low noise amplifier (LNA) 2, a receiving mixer 3, a variable gain amplifier 4, an IF (Intermediate Frequency) mixer 5, an analog/digital (A/D) converter 6, an FIR (Finite Impulse Response) filter 7, a demodulator 8, and a baseband portion 11.
A burst signal in each frame of PHS received by antenna 1 is subjected to low noise amplification in LNA 2, and supplied to receiving mixer 3. Receiving mixer 3 mixes the signal received from LNA 2 with a local oscillation signal received from a local oscillation circuit (not shown), and subjects the mixed signal to frequency conversion to obtain an analog received signal within a predetermined frequency band.
The analog received signal is amplified by variable gain amplifier 4. The wireless receiving device further includes an AGC processing portion 9, a feedback portion 12, and a digital/analog (D/A) converter 13, as portions to control a gain of variable gain amplifier 4.
The analog received signal amplified by variable gain amplifier 4 is supplied to IF mixer 5, and converted to an analog received signal having an intermediate frequency (IF) lower than a radio frequency (RF). Further, the analog received signal is converted to a digital signal by A/D converter 6. The digital signal is supplied to FIR filter 7 for band limiting.
An output of FIR filter 7 is supplied to demodulator 8 and AGC processing portion 9. The functions of demodulator 8 and AGC processing portion 9 are implemented via software, using a digital signal processor (DSP) 10.
For the output signal of FIR filter 7, demodulator 8 performs demodulation processing according to a predetermined modulation scheme (such as π/4 QPSK (Quadrature Phase Shift Keying) scheme). Further, an output of demodulator 8 is supplied to baseband portion 11 to be subjected to predetermined signal processing (such as synchronous processing, propagation path estimation, adaptive array weight estimation, and the like).
AGC processing portion 9 monitors the supplied digital output of FIR filter 7, and generates a control output for adjusting a variable gain of variable gain amplifier 4 such that a power level of the analog received signal in the relevant frame converges to a predetermined level, for output to a gain control input of variable gain amplifier 4.
More specifically, as shown in FIG. 3, the control output of AGC processing portion 9 passes through feedback portion 12, and is converted to an analog control signal by D/A converter 13. In response to the analog control signal, the gain of variable gain amplifier 4 is adjusted.
In this manner, in a conventional wireless receiving device, AGC processing portion 9 receives a burst signal in each received frame via FIR filter 7 to measure its received power level information, and uses the information for gain adjustment of variable gain amplifier 4. Converging the level of the analog received signal to a predetermined level in each frame by means of the AGC operation described above generally requires a time period of around several symbols from a starting point of each frame. For example, in the mobile communications system such as PHS, the required time period is around 5 symbols.
In a signal format for PHS, for example, the leading part of each frame has a known signal section including a preamble (PR), a unique word (UW), and the like, following a burst transient response period (R) corresponding to a transient state in which the burst signal is rising. A known signal of the known signal section is used to perform a variety of signal processing described above.
Thus, the AGC operation is required to be completed before the known signal section starts. To explain the reason, assume a case where the time period required for the AGC operation extends to part of the known signal section.
During the time period required for the AGC operation described above, several leading symbols of a digital signal in each frame obtained by subjecting the output of variable gain amplifier 4 to digital conversion by A/D converter 6 may have false amplitude values. Accordingly, if this time period extends to part of the known signal section, the relevant part of the known signal section will contain a digital signal having a false amplitude value. When the digital signal has an inappropriate amplitude value, a symbol point cannot be recognized correctly, and thus a reception error occurs in the above signal processing. Therefore, to avoid such a reception error, the AGC operation should be completed before the starting point of the known signal section.
Although not shown, FIR filter 7 in FIG. 3 includes delay elements arranged in a plurality of stages corresponding to respective tap coefficients representing a filter characteristic. Consequently, there arises a delay of a predetermined time amount between the timing when variable gain amplifier 4 receives the received signal and the timing when AGC processing portion 9 receives the received signal via FIR filter 7. This time amount is generally around 2 symbols in a wireless receiving system such as PHS.
Therefore, the AGC operation further requires a time period equivalent to the delay amount of FIR filter 7, resulting in a reception error due to the inappropriate amplitude value described above in the leading part of the received signal.
However, even if a digital signal having a false amplitude value is included in part of the known signal section, the frequency of occurrence of a reception error during signal processing is relatively low when the well-known π/4 QPSK scheme is employed as the modulation scheme.
Since this scheme makes a determination based on only a phase component of a digital signal, a symbol point is recognized correctly even if the digital signal has an inappropriate amplitude value, and there is low possibility of causing a reception error in signal processing in a later stage.
Recent mobile communications systems have been required to achieve higher-quality, larger-volume transmission as in data communications when compared to conventional voice communications, and modulation schemes using more values than the π/4 QPSK scheme have been under consideration for application.
As an example of a multi-valued modulation scheme, a well-known 16QAM (Quadrature Amplitude Modulation) scheme has already been practically used in a certain type of data communications. In the 16QAM scheme, a symbol point of a received signal corresponds to one of a total of 16 signal points in a plane with a coordinate system, in which four signal points are arranged in a lattice shape for each quadrant in a plane with IQ coordinates. That is, in this scheme, a symbol point is determined based on both a phase component and an amplitude component of a digital signal.
Therefore, when the 16QAM scheme is employed as a modulation scheme for PHS, if the digital signal has an inappropriate amplitude value, a symbol point is misrecognized as another symbol point having the same phase and a different amplitude value, causing a reception error in signal processing in a later stage.
Further, in the mobile communications system such as PHS, the known signal section is limited to a short section of several symbols in the leading part of each frame, in accordance with a signal format standard, in order to guarantee sufficient transmission data capacity. Consequently, when a multi-valued modulation scheme such as the 16QAM scheme is employed, an amplitude value becomes false in a large section within the known signal section, causing a reception error during digital signal processing.